The present invention is generally directed to a fluid flow control valve assembly and in particular to such an assembly which meters hydraulic fluid from a cylinder port at a constant rate for lowering a load at a corresponding constant rate which has been raised by the cylinder.
Hydraulic systems for raising and lowering heavy loads are well known in the art. Such systems may include, for example, a hydraulic cylinder having a moveable piston disposed therein which is connected at one end to a load to be raised and lowered by suitable linkages. The cylinder usually includes a port below the piston and as hydraulic fluid is forced into the cylinder through the port under pressure, the piston is forced upwardly by the fluid within the cylinder to in turn cause the load to be raised. To lower the load it is necessary to exhaust the hydraulic fluid from the cylinder. Because heavy loads generate a great amount of inertia as they are lowered, it is necessary to provide a valve system which meters the hydraulic fluid being exhausted from the cylinder at a constant rate to assure that the load will be lowered at a corresponding constant safe rate. Obviously, metering of the hydraulic fluid in this manner as the load is lowered is necessary to avoid damage to the hydraulic load rasing system and to the load itself as the load is lowered.
Prior fluid flow control valve systems to accomplish this end have generally included a control valve which maintains a constant pressure within the fluid line conducting the exhausted fluid. Such valves are normally open valves, that is to say, at the instant the hydraulic fluid is first allowed to flow from the cylinder, a great influx of hydraulic fluid impinges upon the normally opened pressure control valve. Such valves also include a fluid flow restricting orifice which is integral to the member within the valve which is instrumental in maintaining the constant fluid pressure. As a result, when the pressure control valve receives the initial influx of exhausted hydraulic fluid, it overcompensates responsive to the influx of fluid and reduces the flow rate of the fluid to a greater extent than necessary. The difference in pressure resulting from the initial overcompensation causes the control valve to again overcompensate and to allow an excessive amount of fluid to flow from the cylinder. This cycle tends to repeat itself many times before a constant flow rate is obtained. As a result, loads are lowered in a jerky or discontinuous manner which creates a hazard when the heavy loads are abruptly halted during their descent.
It is therefore a general object of the present invention to provide a new and improved fluid flow control valve system.
It is a more particular object of the present invention to provide a new and improved fluid flow control valve system for metering fluid from a cylinder port at a constant desired rate for lowering a load at a corresponding constant rate which has been raised by the cylinder.
It is more particular object of the present invention to provide such a system which includes a normally closed pressure compensating valve which gradually opens responsive to the initial flow of fluid from the hydraulic cylinder for gradually increasing the rate of fluid flow from the cylinder until a desired fluid flow rate is obtained and to thereafter maintain the fluid flow rate constant.